c - 从二进制文件中读取随机结构数组元素

Question

我有一个非常大的数组(100k)，其中元素随机放置，并以数组索引作为主键。我试图找出一种使用索引键作为偏移量使用 fseek 访问数组元素的方法。由于某种原因，无论我给出什么偏移量，我都会得到空值。以下是问题的简单表述。

感谢帮助。抱歉，如果这是转发。我在任何地方都找不到类似的问题。

 #include <stdlib.h>
 #include <stdio.h>

 typedef struct student
 {
   char *name;
   int marks;
 }student;

 student class[] = {
   [5]{"Jack",34},
   [12]{"Jane",56},
   [53]{"Joe",72}
 };

 main()
 {
   FILE *f, *f1;
   student rec;

   f = fopen("student.data", "wb");
   fwrite(class, sizeof(student), sizeof(class), f);
   fclose(f);

   if((f1 = fopen("student.data","rb"))==NULL)
   {
     printf("\nError in Opening File\n");
     exit(0);
   }

   /* I want to  seek to 12th element in the array and print 'Jane' */
   fseek(f1, sizeof(student) * 12, SEEK_SET);
   fread(&rec, sizeof(student), 1, f1);

   printf("Name:%s\n", rec.name);

   fclose(f1);
 }

Answer 1

你的例子对我有用。我唯一需要更改的是在类数组初始化中使用双引号而不是单引号:

cat student.c; gcc student.c; ./a.out

给出以下输出:

 #include <stdlib.h>
 #include <stdio.h>

 typedef struct student
 {
   char *name;
   int marks;
 }student;

 student class[] = {
   [5]{"Jack",34},
   [12]{"Jane",56},
   [53]{"Joe",72}
 };

 main()
 {
   FILE *f, *f1;
   student rec;

   f = fopen("student.data", "wb");
   fwrite(class, sizeof(student), sizeof(class), f);
   fclose(f);

   if((f1 = fopen("student.data","rb"))==NULL)
   {
     printf("\nError in Opening File\n");
     exit(0);
   }

   /* I want to  seek to 12th element in the array and print 'Jane' */
   fseek(f1, sizeof(student) * 12, SEEK_SET);
   fread(&rec, sizeof(student), 1, f1);

   printf("Name:%s\n", rec.name);

   fclose(f1);
 }
Name:Jane